Method of Manufacturing Gypsum Board with Improved Fire Resistance

ABSTRACT

The present disclosure is directed to a method of manufacturing gypsum board as well as the resulting gypsum board. The method comprises providing a gypsum slurry with a shrinkage-reducing additive comprising a metal salt of an acid, a silica, or a mixture thereof. As a result, the gypsum board includes gypsum and a shrinkage-reducing additive comprising a metal salt of an acid, a silica, or a mixture thereof. The gypsum board exhibits an area shrinkage of less than 10%.

RELATED APPLICATIONS

The present application claims priority to and the filing benefit toU.S. Provisional Patent Application No. 62/449,484 having a filing dateof Jan. 23, 2017 and which is hereby incorporated by reference in itsentirety.

BACKGROUND OF THE INVENTION

Gypsum wallboards have been used as fire resistant building materialsfor many years. Generally, the fire resistance of the board is derivedfrom the loss of crystalline water (“endothermic dehydration”) presentin the gypsum molecular structure. However, when exposed to hightemperatures, gypsum boards have a tendency to shrink. Such shrinkagecan also result in cracks or voids to form within the core. In turn,this can affect the mechanical integrity and strength of the board andalso create openings that may allow fire or hot air to penetrate throughthe board and/or a wall assembly containing such board. This can resultin increased heat transfer and ultimately, failure of the board and/orthe wall assembly.

As a result, a need exists for providing a gypsum board with improvedresistance. In particular, a need exists for providing a gypsum boardwith reduced shrinkage and improved mechanical strength, for instancewhen exposed to high temperatures.

SUMMARY OF THE INVENTION

Aspects and advantages of the invention will be set forth in part in thefollowing description, or may be apparent from the description, or maybe learned through practice of the invention.

In accordance with one embodiment of the present invention, a method formanufacturing a gypsum board is disclosed. The method comprisesproviding a gypsum slurry with a shrinkage-reducing additive comprisinga metal salt of an acid, a silica, or a mixture thereof. The gypsumboard exhibits an area shrinkage of less than 20%.

In accordance with another embodiment of the present invention, a gypsumboard is disclosed. In particular the gypsum board includes gypsum and ashrinkage-reducing additive comprising a metal salt of an acid, asilica, or a mixture thereof. The gypsum board exhibits an areashrinkage of less than 20%.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdescription and appended claims. The accompanying drawings, which areincorporated in and constitute a part of this specification, illustrateembodiments of the invention and, together with the description, serveto explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth in the specification, which makes reference to the appendedfigures, in which:

FIG. 1 is a graphical representation of a Differential Scanningcalorimetry (DSC) curve of temperature (° C.) and weight loss of aconventional gypsum sample;

FIG. 2 is a data plot of a typical E119 test measuring time andtemperature of a conventional gypsum board;

FIG. 3 is a Thermal Mechanical Analyzer (TMA) plot of a lengthdimension, percent change in dimension and temperature of a conventionalgypsum sample;

FIG. 4 is a comparison of Scanning Electron Microscope (SEM) images ofgypsum crystal morphology when exposed to different temperatures;

FIG. 5 is a Thermal Mechanical Analyzer (TMA) plot of the percent changein dimension and temperature of a gypsum sample and a sample with 1%colloidal silica in accordance with one aspect of the presentdisclosure; and

FIG. 6 is a Scanning Electron Microscope (SEM) image of gypsum crystalscontaining colloidal silica.

DETAILED DESCRIPTION OF THE INVENTION

Reference now will be made in detail to embodiments of the invention,one or more examples of which are illustrated in the drawings. Eachexample is provided by way of explanation of the invention, notlimitation of the invention. In fact, it will be apparent to thoseskilled in the art that various modifications and variations can be madein the present invention without departing from the scope or spirit ofthe invention. For instance, features illustrated or described as partof one embodiment can be used with another embodiment to yield a stillfurther embodiment. Thus, it is intended that the present inventioncovers such modifications and variations as come within the scope of theappended claims and their equivalents.

Generally speaking, the present invention is directed to a method ofmanufacturing gypsum board as well as the resulting gypsum board. Themethod comprises providing a gypsum slurry with a shrinkage-reducingadditive. The shrinkage-reducing additive may include a metal halide, ametal salt of an acid, a silica, or any combination thereof. Suchshrinkage-reducing additives may also be present in the final gypsumboard.

The present inventors have discovered that such shrinkage-reducingadditives can be employed to be present at the gypsum crystal interfacesin order to minimize the shrinkage of the resulting board. Alternativelyor simultaneously, the shrinkage-reducing additives can cause a defectin the gypsum crystal growth during the crystal growth stage at hightemperatures. Nevertheless, without intending to be limited by theory,the present inventors have discovered a manner in which a gypsum boardcan have a lower shrinkage when exposed to high temperatures.

As determined herein, the shrinkage can be measured via thermalmechanical analyzer (TMA). The test is conducted using the E119 ramprates with a sample size of 5 mm by 5 mm by 11 mm (11 mm thickness).Using TMA, a change in dimension, in particular thickness can bedetermined and as measured herein, the shrinkage and thickness is basedon the values at 950° C. Without the shrinkage-reducing additive asdisclosed herein, a gypsum board (i.e., one containing gypsum, theshrinkage-reducing additive, and other additives) may have an averageshrinkage of greater than 20%. However, by employing ashrinkage-reducing additive as disclosed herein, the gypsum board mayexhibit a shrinkage of 20% or less, such as 16% or less, such as 15% orless, such as 14% or less, such as 12% or less, such as 10% or less,such as 8% or less, such as 6% or less, such as 5% or less. The gypsumboard may have an average shrinkage of greater than 0%, such as 0.5% ormore, such as 1% or more, such as 2% or more, such as 3% or more, suchas 5% or more, such as 10% or more. Such percentages are based on theinitial thickness.

As another means for determining the effect of the shrinkage-reducingadditive on gypsum, the shrinkage can be measured by determining an areashrinkage (i.e., of a face instead of a thickness). For such areashrinkage, it can be determined utilizing a cast gypsum bar (i.e., onecontaining only gypsum and the shrinkage-reducing additive) havingdimensions of 1″×1″×11.25″ (face or back of 1″×11.25″ and a thickness of1″), drying the bar at 45° C. until a constant mass is obtained, andconditioning the bar at 70° F. and 50% RH for 12 hours. Afterconditioning, the bars are placed into a muffle furnace and quicklyramped according to ramp rates as defined in ASTM E119 to a temperatureof about 950° C. The percentage change in shrinkage is determined bycomparing the area after heating with the initial 1″×11.25″ area.

In this regard, as illustrated in Table 2 and 3 of the examples, aconventional gypsum bar not containing a shrinkage-reducing additive asdisclosed herein or any other additives may have an area shrinkage of14-17%. Meanwhile, shrinkage-reducing additives may be employed toprovide a reduction in the amount of shrinkage. For instance, suchshrinkage-reducing additives may provide a gypsum board or bar thatexhibits an area shrinkage of 20% or less, such as 17% or less, such as15% or less, such as 14% or less, such as 13% or less, such as 12% orless, such as 10% or less, such as 8% or less, such as 6% or less, suchas 4% or less, such as 3% or less. The area shrinkage may be 1% or more,such as 2% or more, such as 3% or more, such as 5% or more, such as 7%or more, such as 9% or more. Such percentages are based on the initialarea.

The gypsum board may be produced according to any method generally knownin the art. In this regard, the general method of making a gypsum boardmay not necessarily be limited by the present invention.

In general, the gypsum board includes a gypsum core that is producedfrom a gypsum slurry that is prepared as a mixture of dry and wetingredients. The dry ingredients include primarily calcium sulfatehemihydrate and may include one or more additives that are known in theart, such as fiberglass, set accelerators, fillers (e.g., vermiculite),crystal modifiers (e.g., boric acid) and/or binders (e.g., starch).

The wet ingredients may include water and may include one or moreadditional components that are known in the art, such as paper pulp,potash, dispersants, set retarders, polymers, wax emulsion, silicone orsiloxanes, surfactants, and thickening agents. Additional “gaugingwater” may be added to the gypsum slurry to achieve the desiredflowability of the slurry. The dry ingredients, pulp paper solution andgauging water may comprise the basic chemical components of the gypsumcore. In addition, it should be understood that the water to stucco(i.e., calcined gypsum) ratio is not necessarily limited. For instance,while it may be greater than 1 in one embodiment, in another embodiment,it may be 1 or less, such as 0.9 or less.

The dry ingredients are typically mixed in a high speed mixingapparatus. The premix of dry ingredients, the wet ingredients and anygauging water are combined in a mixer (e.g., a pin mixer) to form thegypsum slurry. The gypsum slurry is commonly output from the mixer to acanister and then discharged through an outlet chute or “boot”.

An aqueous foam may also be added to the gypsum slurry, to enhance thefluidity of the slurry and control the core density and weight of thegypsum board. The foam may comprise a mixture of foam water, a foamingagent (e.g., soap) and air, and can be pre-generated using variousmechanical foam generation devices that are known in the art. The foammay be added to the gypsum slurry in the mixer or in the canister whereadditional mixing may occur.

Thereafter, a boot deposits and spreads the gypsum slurry onto a moving,continuous sheet of bottom (back) facing material. A moving, continuoussheet of top (face) facing material is placed on the gypsum slurry, tosandwich the slurry between the top and bottom facing materials and formthe board. The board passes through a forming station which shapes theboard into the desired thickness and width. Although the facing materialis described as paper, other materials known in the art may be used as afacing material, such as fiberglass mat.

The board travels along a belt line for several minutes, during whichtime the stucco and water rapidly undergo a rehydration reaction and theboard stiffens and sets into a solid form. In the rehydration reaction,the calcium sulfate hemihydrate is hydrated to form crystals of calciumsulfate dihydrate, according to the following equation:

CaSO₄·0.5H₂O+1.5H₂O→CaSO₄·2H₂O+heat

The boards are then cut into a desired length and dried in a continuouskiln to evaporate excess (free) water, while the chemically bound wateris retained in the newly formed gypsum crystals. In this regard, thegypsum board now contains an interlocking matrix of gypsum that isformed from stucco, water, and various other additives

In general, the fire resistance of gypsum board is derived from the lossof crystalline water present in the gypsum molecular structure. Gypsumcontains two moles of crystalline water in its molecular structure, orapproximately 21% water by weight, depending on purity. When exposed tohigh temperatures, gypsum goes through two principal dehydrationreactions. In the first reaction, gypsum undergoes a dehydrationreaction to form calcium sulfate hemihydrate.

CaSO₄·2H₂O+Heat→CaSO₄·0.5H₂O+1.5H₂O  (Reaction 1) (Gypsum) (Hemihydrate(bassanite))

In the second reaction, the calcium sulfate hemihydrate is furtherdehydrated and converted to the soluble form of calcium sulfateanhydrite.

CaSO₄·0.5H₂O+Heat→CaSO₄(AIII)+0.5H₂O  (Reaction 2) (Hemihydrate(bassanite)) (Soluble Anhydrite)

The dehydration reactions are endothermic reactions that absorb energyfrom the surroundings, usually in the form of heat. The endothermicreactions hold the gypsum board at a constant temperature until fullydehydrated, which slows the temperature rise of the gypsum duringexposure to fire. The release of water in these dehydration reactionsfurther delays the progress of fire.

Once the dehydration is completed, the temperature of the gypsum boardincreases and the calcium sulfate anhydrite transforms from the solubleform to the insoluble form (AII). This transformation is accompanied bya transition from a rhombohedral to an orthorhombic crystallinestructure, which results in a denser mass and a dimensional contraction(shrinkage) of the gypsum board.

As discussed in Examples 1-4 below, FIGS. 1-3 provide informationregarding the weight loss of a conventional gypsum sample as illustratedusing DSC (FIG. 1), the time/temperature test in accordance with ASTME119 (FIG. 2), and the change in dimension (i.e., thickness) as afunction of temperature as determined according to TMA (FIG. 3).Meanwhile, FIG. 4 provides SEM images of gypsum crystal at varioustemperatures. These images show the transformation of gypsum crystals ina conventional gypsum boards a function of temperature. For instance, at250° C., the gypsum crystals maintain their initial shape and size;however, the structure becomes more porous with increasing numbers offissures and cracks typical of dehydration. At 450° C., the gypsumcrystals maintain the needle-like shape and size, similar to thestructure shown at 250° C.; however, the number of observed fissures andcracks is further increased. At 950° C., the gypsum crystals lose theirneedle-like morphology and transform into a smooth, dense surface. Thismorphology change is attributed to the sintering phenomenon.

The present inventors have discovered that by employing theshrinkage-reducing additives disclosed herein, such sintering phenomenonmay be minimized. For instance, such shrinkage-reducing additives can beincorporated in order to minimize the extent to which the gypsumcrystals are sintered. In addition, the inclusion of suchshrinkage-reducing additive can allow for the formation of a defect inthe crystal growth. As a result, the gypsum crystals are able tomaintain their integrity for a longer period of time at highertemperatures without resulting in excessive shrinkage which in turnwould result in failure of the board at such high temperatures and whentested in accordance with ASTM E119. Thus by minimizing the shrinkage,the gypsum board can have improved fire resistance as well as improvedmechanical integrity and strength.

As a result, the specific surface area of the gypsum at a temperature of950° C. may be about 40% or more, such as about 50% or more, such asabout 70% or more, such as about 80% or more, such as about 90% more,such as about 100% or more, such as about 125% or more, such as about150% or more, such as about 200% more the specific surface area of thegypsum at a temperature of 22° C. Without such shrinkage-reducingadditive, the specific surface area of the gypsum at a temperature of950° C. may be less than 40% of the specific surface area of the gypsumat a temperature of 22° C.

In order to provide the desired effect, the shrinkage-reducing additivemay have a particular size. For instance, the shrinkage-reducingadditive may have a particle size of 200 micrometers or less, such as150 micrometers or less, such as 100 micrometers or less, such as 75micrometers or less, such as 50 micrometers or less, such as 40micrometers or less, such as 25 micrometers or less, such as 15micrometers or less, such as 10 micrometers or less, such as 5micrometers or less, such as 1 micrometer or less, such as about 900nanometers or less, such as about 800 nanometers or less, such as about600 nanometers or less, such as about 500 nanometers or less, such asabout 300 nanometers or less, such as about 200 nanometers or less, suchas about 100 nanometers or less, such as about 50 nanometers or less.The shrinkage-reducing additives may have a particle size of 5nanometers or more, such as 10 nanometers or more, such as 20 nanometersor more, such as 30 nanometers or more, such as 40 nanometers or more,such as 50 nanometers or more, such as 100 nanometers or more, such as250 nanometers or more, such as 500 nanometers or more, such as 750nanometers or more, such as 1 micrometer or more, such as 5 micrometersor more, such as 10 micrometers or more, such as 20 micrometers or more,such as 50 micrometers or more, such as 100 micrometers or more. In oneembodiment, such particle size may be an average particle size of theshrinkage-reducing additive.

In one embodiment, in order to provide a shrinkage-reducing additive atthe gypsum crystal interface, the shrinkage-reducing additive may have aparticular size. In this regard, the shrinkage-reducing additive may beone having a particle size of less than 1 micrometer, such as about 900nanometers or less, such as about 800 nanometers or less, such as about600 nanometers or less, such as about 500 nanometers or less, such asabout 300 nanometers or less, such as about 200 nanometers or less, suchas about 100 nanometers or less, such as about 50 nanometers or less.The shrinkage-reducing additives may have a particle size of 5nanometers or more, such as 10 nanometers or more, such as about 20nanometers or more, such as about 30 nanometers or more, such as about40 nanometers or more, such as about 50 nanometers or more, such asabout 100 nanometers or more, such as about 250 nanometers or more. Inone embodiment, such particle size may be an average particle size ofthe shrinkage-reducing additive.

As indicated herein, a shrinkage-reducing additive is employed in thegypsum board. The shrinkage-reducing additive may include a metal saltof an acid, a silica, or any combination thereof. In one embodiment, theshrinkage-reducing additive may include a metal salt of an acid. Inanother embodiment, the shrinkage-reducing additive may include asilica.

In one embodiment, the shrinkage-reducing additive may include a metalsalt of an acid, such as an organic acid. For instance, the salt may bea sulfate, a carbonate, a phosphate, or a mixture thereof. In oneembodiment, the salt may be a sulfate. In another embodiment, the saltmay be a carbonate. In another embodiment, the sale may be a phosphate.

In addition, the metal may be an alkaline earth metal or a transitionmetal. In one embodiment, the metal may be an alkaline earth metal. Inanother embodiment, the metal may be a transition metal. For instance,the metal may include, but is not limited to, strontium, barium, iron,copper, nickel, titanium, zirconium, manganese, cobalt, silver,aluminum, etc. In one embodiment, the metal may include barium, iron, ora mixture thereof. In one particular embodiment, the metal may includebarium. In another particular embodiment, the metal may include iron.

In one embodiment, the shrinkage-reducing additive may be a metalsulfate. In particular, the shrinkage-reducing additive may includebarium sulfate, iron sulfate, or a mixture thereof In one particularembodiment, the shrinkage-reducing additive may include barium sulfate.In another particular embodiment, the shrinkage-reducing additive mayinclude iron sulfate. In one embodiment, the shrinkage-reducing additivedoes not include calcium sulfate.

In one embodiment, the shrinkage-reducing additive may be a metalcarbonate. In particular, the shrinkage-reducing additive may includebarium carbonate, iron carbonate, or a mixture thereof. In oneparticular embodiment, the shrinkage-reducing additive may includebarium carbonate. In another particular embodiment, theshrinkage-reducing additive may include iron carbonate.

In one embodiment, the shrinkage-reducing additive may be a metalphosphate. In particular, the shrinkage-reducing additive may includebarium phosphate, iron phosphate, or a mixture thereof. In oneparticular embodiment, the shrinkage-reducing additive may includebarium phosphate. In another particular embodiment, theshrinkage-reducing additive may include iron phosphate.

In one embodiment, the shrinkage-reducing additive may be formed from aprecursor shrinkage-reducing additive. For instance, the precursorshrinkage-reducing additive may be provided to the slurry and undergo areaction to form the shrinkage-reducing additive. In one embodiment, theprecursor shrinkage-reducing additive may include a metal halide. Forinstance, the metal of the metal halide may be any of the aforementionedmetals referenced above. The halide may generally be any halide. Thehalide may be any halide, such as fluoride, chloride, bromide, iodide,or a mixture thereof. However, in one embodiment, the halide may be achloride.

In one embodiment, when the shrinkage-reducing additive is a metalsulfate, such sulfate may be provided in the gypsum board via theprecursor shrinkage-reducing additive. Such precursor shrinkage-reducingadditive may be a metal halide wherein the metal of the metal halide isthe same as the metal of the metal sulfate. In this regard, the metalhalide may be provided to the slurry and via the existing sulfate (e.g.,calcium sulfate) present within the slurry, a reaction may occur betweenthe metal halide and the calcium sulfate to yield the metal sulfateshrinkage-reducing additive. For instance, a barium chloride precursorshrinkage-reducing additive may be employed to react with the calciumsulfate present in the slurry to yield barium sulfate. As anotherexample, an iron chloride precursor shrinkage-reducing additive may beemployed to react with the calcium sulfate present in the slurry toyield an iron sulfate.

The present inventors have discovered that such method allows for thesynthesis of a precipitated metal sulfate. Such reaction allows for thesynthesis of precipitated metal sulfate having a much smaller particlesize than conventional and readily available metal sulfates, which haveparticle sizes in the micrometer-scale range. In addition, conversion ofsuch conventional and readily available metal sulfates to such ananometer-scale as required may not be practical. In this regard, thepresent inventors have discovered that providing a shrinkage-reducingadditive via the aforementioned means can allow for even a furtherreduction in the amount of shrinkage.

In one embodiment, the shrinkage-reducing additive may be a silica. Thesilica may be a colloidal silica, a precipitated silica, or a mixturethereof. In one embodiment, the silica may be a colloidal silica. Inanother embodiment, the silica may be a precipitated silica. Withoutintending to be limited by theory, the silica may be chemically inertunlike the formation of the aforementioned metal salts of the acidswhich may be formed via an in situ reaction by the incorporation of aprecursor shrinkage-reducing additive, such as a metal halide.

In one embodiment, the shrinkage-reducing additive may be colloidalsilica. As generally known in the art, colloidal silica may be asuspension of fine, amorphous, generally spherical silica particles.Generally, colloidal silica may also be nonporous. Generally, the silicaparticles in the colloidal silica may be monodisperse with respect toparticle size. For instance, the deviation in particle size may be 10%or less, such as 5% or less, such as 3% or less, such as 2% or less,such as 1% or less. In one embodiment, the colloidal silica may have apH of 7 or more. The colloidal silica may have a solids content of 75%or less, such as 60% or less, such as 50% or less, such as 45% or less,such as 35% or less, such as 30% or less and 5% or more, such as 10% ormore, such as 20% or more, such as 30% or more, such as 40% or more.

In one embodiment, the shrinkage-reducing additive may be precipitatedsilica. As generally known in the art, precipitated silica is producedby precipitation from a solution containing silicate salts. Generally,precipitated silica may be porous and may be present as an agglomerate.The precipitated silica may have a D50 of 500 nanometers or more, suchas 1 micrometer or more, such as 2 micrometers or more, such as 3micrometers or more, such as 5 micrometers or more, such as 10micrometers or more, such as 15 micrometers or more, such as 25micrometers or more to 100 micrometers or less, such as 75 micrometersor less, such as 50 micrometers or less, such as 40 micrometers or less,such as 25 micrometers or less, such as 20 micrometers or less, such as15 micrometers or less, such as 10 micrometers or less, such as 5micrometers or less, such as 4 micrometers or less, such as 1 micrometeror less. Such aforementioned D50, in one embodiment, may refer to theagglomerate size of the precipitated silica.

In addition to the above, the shrinkage-reducing additive may be onethat does not burn at a temperature of 400° C. or more, such as about500° C. or more, such as about 700° C. or more, such as about 900° C. ormore.

The shrinkage-reducing additives may be present in an amount of about 20wt. % or less, such as about 15 wt. % or less, such as about 10 wt. % orless, such as about 8 wt. % or less, such as about 5 wt. % or less, suchas about 4 wt. % or less, such as about 3 wt. % or less, such as about 2wt. % or less, such as about 1 wt. % or less, based on the weight of thestucco (i.e., calcined gypsum). The shrinkage-reducing additives may bepresent in an amount of about 0.05 wt. % or more, such as about 0.1 wt.% or more, such as about 0.25 wt. % or more, such as about 0.5 wt. % ormore, such as about 0.75 wt. % or more, such as about 1 wt. % or more,such as about 2 wt. % or more, such as about 4 wt. % or more, such asabout 5 wt. % or more, based on the weight of the stucco (i.e., calcinedgypsum). In one embodiment, the shrinkage-reducing additive may bepresent in a final gypsum board in the aforementioned amounts whereinthe amount is based on the amount of gypsum in the board.

It should be understood that such shrinkage-reducing additives may beused alone or in any combination. For instance, combinations may beeffective at providing a synergistic effect to reduce the shrinkage.

In addition, it should be understood that the point of the process inwhich the shrinkage-reducing additives are incorporated is notnecessarily limited. That is, the shrinkage-reducing additives may beincorporated into a gypsum slurry at various steps in the manufacturingprocess. In addition, where multiple shrinkage-reducing additives areemployed, such shrinkage-reducing additives may be added to the gypsumslurry at the same or at different steps of the manufacturing process.However, in one preferred embodiment, the shrinkage-reducing additivesare added to the gypsum slurry by mixing with the dry ingredients. Forinstance, such shrinkage-reducing additives may be provided in a mixerof the manufacturing process. Alternatively, the shrinkage-reducingadditives may be added in the mill.

The gypsum board produced according to the method disclosed herein mayhave any combination of physical and/or mechanical properties as desiredfor the particular application. In this regard, the gypsum boarddisclosed herein is not necessarily limited by weight, density, or othermechanical properties (e.g., nail pull strength, flexural strength, corehardness, humidified bond load, etc.).

The board may have any weight as generally employed in the art. Forinstance, the board may have a weight of about 2500 lbs/MSF (lbs per onethousand square feet) or less, such as about 2400 lbs/MSF or less, suchas about 2200 lbs/MSF or less, such as about 2000 lbs/MSF or less, suchas about 1800 lbs/MSF or less, such as about 1500 lbs/MSF or less, suchas about 1300 lbs/MSF or less. The board may have a weight of about 1000lbs/MSF or more, such as about 1200 lbs/MSF or more, such as about 1400lbs/MSF or more, such as about 1600 lbs/MSF or more, such as about 1800lbs/MSF or more, such as about 2000 lbs/MSF or more. By about, theweight may vary by +/−5%, such as +/−3%, such as +/−2%, such as +/−1%.

The board may be any thickness as generally employed in the art. Forinstance, in one embodiment, the board may have a thickness of about ¼″.In another embodiment, the board may have a thickness of about ⅜″. Inanother embodiment, the board may have a thickness of about ½″. In afurther embodiment, the board may have a thickness of about ⅝″. Byabout, the thickness may vary by +/−5%, such as +/−3%, such as +/−2%,such as +/−1%.

In one embodiment, the hardness (as determined in accordance with ASTMC473), may be about 9 lb_(f) or more, such as about 10 lb_(f) or more,such as about 11 lb_(f) or more, such as 12 lb_(f) or more, such as 13lb_(f) or more. The hardness may be about 50 lb_(f) or less, such asabout 40 lb_(f) or less, such as about 30 lb_(f) or less, such as about25 lb_(f) or less, such as about 20 lb_(f) or less, such as about 18lb_(f) or less, such as about 15 lb_(f) or less. The aforementionedhardness describes the core hardness, the end hardness, or both. In oneembodiment, the aforementioned hardness describes the core hardness.

In one embodiment, the nail pull strength (as determined in accordancewith C473), may be about 70 lb_(f) or more, such as about 75 lb_(f) ormore, such as about 77 lb_(f) or more, such as about 80 lb_(f) or more,such as about 85 lb_(f) or more, such as about 90 lb_(f) or more, suchas about 100 lb_(f) or more. The nail pull strength may be about 120lb_(f) or less, such as about 110 lb_(f) or less, such as about 100lb_(f) or less, such as about 90 lb_(f) or less, such as about 80 lb_(f)or less.

In one embodiment, the flexural strength in the perpendicular direction(as determined in accordance with ASTM C473) may be about 90 lb_(f) ormore, such as about 100 lb_(f) or more, such as about 110 lb_(f) ormore, such as about 130 lb_(f) or more, such as about 150 lb_(f) ormore, such as about 160 lb_(f) or more, such as about 175 lb_(f) ormore. The flexural strength in the perpendicular direction may be about250 lb_(f) or less, such as about 225 lb_(f) or less, such as about 200lb_(f) or less, such as about 180 lb_(f) or less, such as about 175lb_(f) or less, such as about 150 lb_(f) or less, such as about 125lb_(f) or less.

In one embodiment, the flexural strength in the parallel direction (asdetermined in accordance with ASTM C473) may be about 20 lb_(f) or more,such as about 25 lb_(f) or more, such as about 30 lb_(f) or more, suchas about 40 lb_(f) or more, such as about 50 lb_(f) or more, such asabout 60 lb_(f) or more. The flexural strength in the parallel directionmay be about 80 lb_(f) or less, such as about 70 lb_(f) or less, such asabout 60 lb_(f) or less, such as about 50 lb_(f) or less, such as about40 lb_(f) or less.

In addition, the gypsum board may be manufactured so as to pass thenecessary UL tests for fire-resistance. For instance, the gypsum boardmay pass ASTM E119, which is the standard test method for a fire test ofa building construction and materials.

EXAMPLES Example 1 Dehydration of a Conventional Gypsum Board

The dehydration of a conventional gypsum board was tested bythermogravimetric analysis and differential scanning calorimetry, asshown in FIG. 1. The energy required for complete dehydration of gypsumis about 625 kJ/kg, which is consumed in the separation and evaporationof the water molecules from the gypsum crystals. The dehydration curveshows that endothermic Reactions 1 and 2 take place between about 80° C.to 220° C., with respective peaks at about 160° C. and about 190° C.Once dehydration is complete, the temperature of the gypsum samplecontinues to rise. A third peak is shown between about 350° C. to 380°C., which corresponds to the exothermic transformation from soluble aanhydrite (AIII) to insoluble β anhydrite (AII), and the transition inthe gypsum crystal structure.

Example 2 Fire Test of Conventional Gypsum Board

The industrial fire testing standard ASTM E119 provides strictrequirements for testing wallboard in a controlled testing assembly toensure that products meet certain fire resistance standards.

A conventional gypsum board was subjected to an ASTM E119 fire test.FIG. 2 shows the board temperature profiles during gypsum dehydrationfor both the exposed and unexposed sides of the gypsum board in the wallassembly, along with the furnace temperature ramp. The temperature ofthe gypsum board does not increase appreciably during the dehydrationreaction, particularly on the unexposed board side. However, thetemperature increase resumes when the dehydration reaction is finished,as shown by the temperature rise on the exposed side of the gypsumboard.

Example 3 Thermomechanical Analysis of Conventional Gypsum Board

A conventional gypsum board was subjected to thermomechanical analysis(TMA) to determine the dimensional change with increasing temperature.The dehydration reaction and the transformation from soluble toinsoluble anhydrite results in a series of shrinkages, as shown in FIG.3. The first shrinkage starts at about 120° C. and ends at about 200°C., as a result of the gypsum crystal dehydration. The shrinkage isabout −0.23% from its original thickness, depending upon gypsum purityand impurities. The second shrinkage starts at about 350° C. and isabout 10 times greater than the first shrinkage (−2.3% vs. −0.23%). Thisshrinkage is the result of the change from soluble to insolubleanhydrite and the accompanying change from rhombohedral to orthorhombiccrystal structure. The third shrinkage, and the largest one, starts atabout 600° C. and shows shrinkage of about −23% at about 950° C., whichis a result of the sintering process the insoluble anhydrite goesthrough at higher temperatures.

Example 4

A conventional gypsum board was analyzed by scanning electron microscopy(SEM) at temperatures of 22° C., 250° C., 450° C., and 950° C., as shownin FIG. 4. At 250° C., the gypsum crystals maintain their initial shapeand size; however, the structure becomes more porous with increasingnumbers of fissures and cracks typical of dehydration. At 450° C., thegypsum crystals maintain the needle-like shape and size, similar to thestructure shown at 250° C.; however, the number of observed fissures andcracks is further increased. At 950° C., the gypsum crystals lose theirneedle-like morphology and transform into a smooth, dense surface. Thismorphology change is attributed to the sintering phenomenon. X-raydiffraction (XRD) analysis indicates an orthorhombic crystallinestructure, which is typically seen in insoluble anhydrite.

This transformation is further evidenced by a comparison of the specificsurface area (SSA) of the gypsum samples, as shown in Table 1 below. At22° C., the sample has an SSA of 1.330 m²/g. As the sample is heated,fissures and cracks are introduced, which increase in number and sizewith increased temperature. The SSA of the gypsum sample increases to4.819 m²/g at 250° C. and to 15.595 m²/g at 450° C. At 950° C., thesintering process increases the sample density and thus reduces thesurface area to 0.505 m²/g.

TABLE 1 Specific Surface Area of Conventional Gypsum Based onTemperature Specific Surface Area Sample (m²/g) Cube Heated to 22° C.1.330 Cube Heated to 250° C. 4.819 Cube Heated to 450° C. 15.595 CubeHeated to 950° C. 0.505

Example 5

The shrinkage performance of cast gypsum was determined. The gypsum wasa cast gypsum bar having dimensions of 1″×1″×11.25″ (face or back of1″×11.25″ and a thickness of 1″). The bar was dried at 45° C. until aconstant mass was obtained. The bar was then conditioned at 70° F. and50% RH for 12 hours. After conditioning, the bars were placed into amuffle furnace and quickly ramped according to ramp rates as defined inASTM E119 to a temperature of about 950° C. All of the samples weredimensionally measured before and after heating where the initial1″×11.25″ area was compared to the post heating area.

Table 2 shows the shrinkage-reducing additives, concentration of suchshrinkage-reducing additives, and the percentage of area shrinkage thatoccurred for these shrinkage-reducing additives.

TABLE 2 Shrinkage performance of a gypsum bar with variousshrinkage-reducing additives Amount Shrinkage (%, w/w based areaAdditive on stucco) (%) None (Control) n/a 14-16 Barium chloride 0.9 2.9Barium carbonate 0.9 13.2 Barium sulfate 1.0 16.3 Aluminum phosphate 1.96.6 Iron chloride 1.0 6.3 Iron sulfate 1.0 14.7 Colloidal 12 nm, 40%solids 1.0 7.9 silica 12 nm, 25% solids 1.0 4.6 22 nm, 34% solids 1.05.8 Precipitated D50 of 18 μm 1.0 8.9 silica D50 of 4 μm 1.0 8.2 D50 of2.5 μm 1.0 8.5

Example 6

The shrinkage performance of a gypsum board containing gypsum, ashrinkage-reducing additive, and at least one other additive wasdetermined. The gypsum board was analyzed to determine the areashrinkage as described in Example 5 or via thermomechanical analysis todetermine a dimensional (i.e., thickness) change with increasingtemperature. The gypsum had a sample size of 5 mm by 5 mm by 11 mm. Thesample was dried at 45° C. until a constant mass was obtained. The barwas then conditioned at 70° F. and 50% RH for 12 hours. The ramp ratewas the E119 ramp rate.

Table 3 shows the shrinkage-reducing additives, concentration of suchshrinkage-reducing additives, and the percentage of shrinkage thatoccurred for these shrinkage-reducing additives.

TABLE 3 Shrinkage performance of a gypsum board with variousshrinkage-reducing additives Amount Area Thickness (%, w/w basedShrinkage Shrinkage Additive on stucco) (%) (%) None (Control 1) n/a16.34 — None (Control 2) n/a — 29 Barium chloride 0.4 13.89 — 1.0 11.68— 2.0  7.18 — 5.0 13.20 — Colloidal silica 1.0 — 12

FIG. 5 includes a Thermal Mechanical Analyzer (TMA) plot of the percentchange in dimension and temperature of a gypsum sample and a sample with1% colloidal silica. As can be observed, a significant dimensionalchange for the gypsum control occurs before 950° C. Meanwhile, with thecolloidal silica, such change is shifted to a higher temperature.Furthermore, FIG. 6 provides an SEM image of the gypsum crystalscontaining colloidal silica.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they include structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

1-22. (canceled)
 23. A method for manufacturing a gypsum board, themethod comprising: providing a gypsum slurry and a shrinkage-reducingadditive comprising a silica, wherein the gypsum board exhibits an areashrinkage of less than 20%.
 24. The method according to claim 23,wherein the silica comprises colloidal silica, precipitated silica, or amixture thereof.
 25. The method according to claim 23, wherein thesilica comprises colloidal silica.
 26. The method according to claim 23,wherein the silica comprises precipitated silica.
 27. The methodaccording to claim 23, wherein the shrinkage-reducing additive has aparticle size of less than 1 nanometer.
 28. The method according toclaim 23, wherein the gypsum board exhibits an area shrinkage of lessthan 10%.
 29. A gypsum board comprising: gypsum and a shrinkage-reducingadditive comprising a silica, wherein the gypsum board exhibits an areashrinkage of less than 20%.
 30. The gypsum board according to claim 29,wherein the silica comprises colloidal silica, precipitated silica, or amixture thereof.
 31. The gypsum board according to claim 29, wherein thesilica comprises colloidal silica.
 32. The gypsum board according toclaim 29, wherein the silica comprises precipitated silica.
 33. Thegypsum board according to claim 29, wherein the shrinkage-reducingadditive has a particle size of less than 1 nanometer.
 34. The gypsumboard according to claim 29, wherein the gypsum board exhibits an areashrinkage of less than 10%.